Umm, are you saying you had the PSU's mobo connector plugged into a tester while the 2x2 was plugged into the mobo?! Also, can you describe the problem in more detail? What happens when you press the power button? Do the fans spin?
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David SchwartzJul 9 '12 at 22:26

If the tester says your psu is fine then it may well be that some of the components on your motherboard are dead. Try removing components such as graphics card and hard drives, leaving just CPU and memory and see if it boots or at least powers up.
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Mokubai♦Jul 9 '12 at 22:34

Pull the motherboard out of the PC tower and place it on a anti-static sheet, even the foil that the motherboard comes in will work. Plug in the PSU and see what happends
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DariusJul 9 '12 at 22:34

2

@Darius, those antistatic bags are actually conductive. That's how they dissipate any excess charge. They're not very conductive, but putting a motherboard on one and powering it up isn't a good idea.
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WyzardJul 9 '12 at 23:26

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The 2x2 should be plugged into the tester, as well as the 20/24 plug at the same. None of those plugs should be plugged into the mobo when you test the PSU.
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LogmanJul 9 '12 at 23:34

1 Answer
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This strongly suggests that the power regulation/conditioning circuitry for the CPU (that's what the aux connector is for) is "blown". The same circuitry may sometimes also be used to regulate/route USB power, which may have caused the problem.

Hopefully your CPU/RAM/add-in cards are OK (like PSUs, such circuitry is usually designed to sacrifice itself before letting a short, overcurrent, etc. affect devices downstream.

Unfortunately, yes, you need a new motherboard. (or if under warranty, get this one repaired -- it's a manufacturing defect -- I've had USB shorts happen numerous times when doing embedded device development and while the port may shut down, on a well-designed motherboard a cold reboot will usually bring it back up)

thank you, its a brand new mobo, it should still be under the manufac warranty.
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Mario GarciaJul 9 '12 at 23:51

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@MarioGarcia: Yeah, so long as he doesn't tell them what he did. Partially powering a motherboard is not good for its health.
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David SchwartzJul 10 '12 at 0:15

@DavidSchwartz: "One day i plugged in my printer on the usb and shorted the pc out" is a manufacturing defect. He can't be blamed for testing things out after his motherboard stopped booting up.
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izxJul 10 '12 at 0:44

@izx: The computer was working fine after whatever happened with the printer and the USB.
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David SchwartzJul 10 '12 at 0:47

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And then it just stopped working one fine day...before he tried to troubleshoot it. In any case, all partially powering up a properly-designed motherboard should do is not let it turn on, not blow it up.
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izxJul 10 '12 at 0:52